ASHRAE Journal - September 2019 - 62

COLUMN ENGINEER'S NOTEBOOK
Stephen W. Duda

Selecting and Specifying
Pipe Flow Meters
BY STEPHEN W. DUDA, P.E., BEAP, HBDP, HFDP, FELLOW ASHRAE

In my most recent "Engineer's Notebook" column (May 2019), I discussed how
measuring airflow rates is an important part of most HVAC controls systems and
reviewed pros and cons of several types of duct-mounted airflow measurement
devices in various HVAC applications. This month's column is intended to help
designers and engineers select and specify fluid flow measurement devices for
mounting in HVAC piping systems, which I will call "flow meters" hereinafter.
In my experience, measuring chilled water, heating
hot water, condenser water, or steam flow in HVAC systems is less common than measuring ducted airflow.
Most sequences of operation for hydronic systems control are based on water temperature, static pressure,
valve position, and the like. Other control sequences
merely need to know whether water is flowing or not,
such as with sail switches. But there are certainly cases
where continuous flow rate measurement is specified
or desired, including medical or educational campus
applications where an owner wishes to track and/or
internally bill individual campus buildings for their
HVAC hydronic water use. Many green building evaluation criteria include flow metering as a recommended
step for comparing or trending building energy use
under the philosophy "you can't analyze what you don't
measure."
Therefore, this column reviews multiple distinct nonproprietary types of pipe system flow measurement
devices that are commonly applied in HVAC systems and
are commercially available from multiple manufacturers. Subchapter 7 within Chapter 37 of the 2017 ASHRAE
Handbook-Fundamentals1 includes such a discussion of
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various flow meter types but seems to lack a list of pros
and cons and recommended applications for each. So
this column hopes to expand upon the Handbook chapter and assist the reader in understanding the pros and
cons of each type and ultimately in making an application-based selection.

Electromagnetic Flow Meters
Permanent in-line electromagnetic-type flow meters
(Figure 1) are available for measuring flow both locally and
via a transmitter to send signals to a building automation
system (BAS). These are mounted in the line of pipe, usually joined to the pipe via bolted flanges as if they were
a valve, strainer, or pipe fitting. They feature no moving
parts and are suitable for bidirectional flow measurement. Wetted parts (those parts in contact with the fluid
whose flow is being measured) are typically constructed
of stainless steel or synthetic materials (for good corrosion
resistance in a wide variety of fluid types).
The operating principle for an electromagnetic flow
Stephen W. Duda, P.E., is senior mechanical engineer at Ross & Baruzzini, Inc.,
in St. Louis. He is a member and former chair of TC 9.1, Large Building AirConditioning Systems.

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ASHRAE Journal - September 2019

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